[mypyc] Fix exception reraising when awaiting a future

[p-sawicki]

Fixes mypyc/mypyc#1177.

Fixed incorrect behavior seen when a function compiled with mypyc awaits an expression in which a future raises an exception, eg.

async def inner():
  try:
    return await future_that_raises
  except Exception:
    # do something

async def outer():
  return await inner()

It seems like this case is different from a regular async function raising an exception in that the exception raised by the future is available in the handled exception state seen by the delegating generator (meaning the outer function compiled with mypyc) even when the exception is caught in the inner function.

The delegating generator reraises the exception from the handled exception state in CPy_YieldFromErrorHandle which ends up reraising an already caught exception.

The fix is to also return the function early when the delegated generator throws an exception other than StopIteration (for StopIteration it already returns), meaning res is NULL in line 71 and also in line 80. This is now consistent with the implementation in cpython which does not reraise on exception in the delegated generator (the reraising happens under label throw_here which is reached under different conditions).

[JukkaL]

This was a pretty tricky issue. Thanks for writing detailed test coverage!